1. Field of the Invention
The present invention relates to a color braun tube and more particularly to a shadow mask for sorting colors of electron beams in a color braun tube.
2. Description of Related Art
Typically, a color braun tube, as shown in FIG. 1, comprises a panel 1 coated with an R, G, B fluorescent material 1a on the inner surface in a certain pattern and a funnel 2 which is mated with the panel 1 at the edge of the panel 1 and narrowed backward so as to define a neck portion 2a shaping like a bottle neck.
Within the panel 1, a mask 3 having a plurality of holes (hereinafter, called beam passage holes), e.g., slits or porosities, is fixed to and supported by a frame 4.
The frame 4 is mated with an inner shield 5 for blocking an external earth magnetic field. The neck portion 2a encloses an electron gun 7 for emitting R, G, B electron beams 6. A deflection yoke 8 is provided around the neck portion 2a to deflect the electron beams 6 in horizontal and vertical directions.
In such configured color braun tube, once a video signal is supplied to the electron gun 7, the electron beams 6 are emitted from a cathode of the electron gun 7. The emitted electron beams 6 are controlled, accelerated, and focused by different voltages applied to respective electrodes of the electron gun 7. The electron beams 6 are then horizontally and vertically deflected by a static magnetic field of the deflection yoke 8 and pass through beam passage holes 3a on the mask 3. The beams passing through the holes 3a make the fluorescent material 1a on the inner surface of the panel 1 emit light to produce an image. Various types of masks can be properly applied in accordance with characteristics of braun tubes. Representative masks are a grille mask without bridge protrusions, a slot mask having bridge protrusions and vertically elongated circular beam passage holes, and a dot mask having beam passage holes relatively smaller than those of the slot mask.
As shown in FIG. 2, the grille mask 3 has the largest beam passage holes 3a compared with other mask types, thereby effecting good luminance. Furthermore, since the grille mask 3 does not include bridge protrusions, a moire phenomenon does not occur.
On the other hand, since occupation rate of beam passage holes 3a is higher than that of no-hole portions 3b in the mask, the grille mask 3 is vulnerable in strength.
To make up for the weak point, there is disclosed a mask as shown in FIG. 3, wherein a length of each beam passage hole is shortened, shaping like the slot mask type, and bridge protrusion 3c named false ties are arranged in a staggered fashion on both inner sides of each beam passage hole across the mask (U.S. Pat. No. 4,926,089).
This mask type enhances the strength of the mask by means of the bridge protrusion 3c. However, a size of the bridge protrusion 3c is not clearly defined. This may lead to a problem of decrease in luminance.
The above U.S. Patent describes that it is preferable to select a size of the bridge protrusion 3c such that a black matrix (BM) or bridge shadow is not visible on the screen to the viewer at a normal viewing distance. There is no specific description on the size of the bridge protrusion 3c other than the above description. Therefore, it can be easily noted that the luminance is decreased by an area where the electron beams are blocked by the bridge protrusion 3c. 
Furthermore, the bridge protrusion 3c is formed to supplement the strength of the mask 3, but side effects by the bridge protrusion are not considered.
In addition, it is easily predictable that the bridge protrusion 3c needs a sufficient size to make the mask 3 have an enough strength by means of the bridge protrusion 3c. In this occasion, the large bridge protrusion 3c covers a large area of the fluorescent material 1a and more electron beam is blocked, thereby decreasing the luminance.
If the size of the bridge protrusion 3c is very small, of course, the above problems do not occur. However, as stated above, the U.S. Pat. No. 4,926,089 is provided to enhance the strength of the mask, so it is reasonable to exclude this case.
Alternatively, the slot mask 3, as shown in FIG. 4, has beam passage holes 3a smaller than those in the grille mask bridge protrusions 3d connected to no-hole portions 3b are provided between beam passage holes linearly arranged in a vertical direction.
Compared with the grille mask, the slot mask decreases in luminance because the quantity of transmitting beams is decreased. Moreover, the bridge protrusions 3d causes the moire phenomenon in the slot mask.
The moire phenomenon is not avoidable in masks having bridge protrusions. The moire phenomenon means that portions emitting light by means of electron beams and portions shaded by bridge protrusions 3d alternately and periodically appear on a screen. The moire phenomenon produces fringes in an image displayed on the screen.
Such moire is sufficiently influenced by a vertical pitch (vp) of beam passage holes 3a in the slot mask 3.
At this time, it should be considered that the vertical pitch (vp) of beam passage holes 3a must be designed to be kept at an appropriate level to maintain a desirable strength of the mask 3. Besides, each bridge protrusion 3d should have an appropriate size to manage mechanical characteristics such as external impulse and howl.
The above facts limit the extension of vertical pitch (vp). The limitation of the vertical pitch (vp) extension causes an amount of the electron beams 6 to be blocked, decreasing an area of the fluorescent material 1a emitting the light. This results in reduction of luminance and makes the moire phenomenon more serious.
FIG. 5 is a graph showing a functional relation between a vertical distance of the mask and a strength of the electron beam scanned in the vertical direction. The data values in FIG. 5 are obtained from simulation.
According to the graph, points are irregularly located on a curved line, showing luminance distribution of long wavelengths. This means that the moire is visually recognizable.
In other words, each point depicted in the graph represents moire appearing on the screen. The points located along the curved line other than points corresponding to the turning points from decrease to increase represent the moire produced by other causes than shadow of bridge protrusions 3d. This kind of moire occurs when a value of the pitch of beam passage holes 3a is small. Therefore, the pitch of beam passage holes 3a should be lengthened to reduce the moire.
Accordingly, the present invention is directed to a shadow mask for a color braun tube that substantially obviates one or more of the limitations and disadvantages of the related art.
An objective of the present invention is to provide a shadow mask for reducing or preventing moire by improving a structure of a mask for a color braun tube.
Additional features and advantages of the invention will be set forth in the following description, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure as illustrated in the written description and claims hereof, as well as the appended drawings.
To achieve these and other advantages, and in accordance with the purpose of the present invention as embodied and broadly described, a first embodiment is provided. According to the first embodiment, in a color braun tube including a mask having many beam passage holes over emitting paths of electron beams, a mask for the color braun tube has a vertical pitch of the beam passage holes that is 2.7 and more times as long as a vertical pitch of horizontal electron beam lines scanned on a screen.
According to a second embodiment, in a color braun tube including a mask having many beam passage holes over emitting paths of electron beams, a mask for the color braun tube has a vertical pitch of the beam passage holes that is 3.4 and more times as long as a vertical pitch of horizontal electron beam lines scanned on a screen.
According to a third embodiment, in a slot mask comprising beam passage holes whose length is shorter than a length of each beam passage hole in a grille mask and longer than a length of each beam passage hole in a dot mask and bridge protrusions connected to no-hole portions between the beam passage holes arranged in a vertical direction, a mask for a color braun tube has symmetrical left and right bridge protrusions by separating and isolating each of the bridge protrusions at its center, each of the left and right bridge protrusions covering a part of a fluorescent material including a black matrix.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.